Process for reducing aromatic nitrogen compounds



Unite States 3,063,980 PROCESS FGR REDUCHNG ARGMATIC ()GEN COMPOUNDS Albert Bloom, Summit, and David E. Graham, Westfield, N.J., assignors to General Aniline 8: Film Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed July 9, 1957, Ser. No. 670,647 6 Claims. ((11. 260-205) This invention relates to improvements in the reduction of aromatic nitrogen compounds containing nitrogen in reducible form as a nuclear substituent in an aromatic ring. More particularly, this invention relates to improvements in the process of effecting reduction of such nitrogen compounds under alkaline conditions, and specifically, under the reductive effect of metal alcoholates whereby reduction products at a stage of oxidation lower than azoxy are produced.

The alkaline reduction of aromatic nitrogen compounds which contain nitrogen in a reducible state is Well known. For example, it is well known that aromatic nitro, nitroso, azo, azoxy and hydroxylamino compounds may be reduced by alkaline reducing agents and that the final stage of reduction is a function of the reducing agent used, the reaction conditions employed, and the absence or presence of promoters and the like.

During the alkaline reduction of an aromatic nitro compound, the following reduction products listed in order of, and by degree of reduction may be obtained:

expense due to the difliculty of recovery for reuse of these promoters, and also due to the fact that they rapidly lose the desired activity and function of carrying the reaction beyond the hydrazo state. An additional deficiency which lies with the use of these promoters is the extremely slow reaction time required to bring about a reduction from a nitro to a hydrazo state.

It is therefore an object of this invention to provide a process whereby the reductive power of metal alcoholates may be greatly increased when they are employed in efiecting the reduction of aromatic nitrogen compounds containing nitrogen in reducible form and at a stage of.

oxidation higher than the hydrazo stage.

It is a further object of this invention to provide a process whereby aromatic nitrogen compounds containing nitrogen in a reducible form and at a stage of oxidation higher than the hydrazo stage may be reduced with metal alcoholates in an economical manner.

It is a still further object of this invention to provide a process whereby aromatic nitrogen compounds containing nitrogen in a reducible state and at a stage of oxidation higher than the hydrazo stage may be reduced with metal alcoholates to give outstanding yields of products having an oxidation level lower than azoxy.

Other objects will appear hereinafter as the description proceeds. According to this invention, the foregoing objects are accomplished by carrying out the reduction of aromatic nitrogen compounds containing reducible nitrogen With metal alcoholates, and particularly alkali ARNOz ARNO ARNHOH ARN NAR ARN=NAR nitro nitroso hydroxylamino azoxy ARNHNHAR ARNH2 hydrazo amine It is well known and has been established that under the usual conditions the reduction of an aromatic nitro compound by means of metal alcoholates, and especially an alkali metal alcoholate such as sodium methylate, gives rise to the azoxy compound as the sole reduction compound. Such reductions are usually carried out by heating nitrobenzene with alcoholic caustic alkali at the boiling point of the mixture employing reflux conditions at atmospheric pressure. The major product, as indicated above, is azoxybenzene although minor amounts of aniline are produced. It has thus been recognized that these alkali metal alcoholates are not sufficiently strong reducing agents to carry the reduction beyond the azoxy stage under the above described conditions. It is also known that the azoxy compounds so produced may be reduced still further to azobenzene and to hydrazobenzene by the use of stronger agents such as aluminum and caustic, or zinc and alkali, or by operating at elevated temperatures (140 to 180 C.) and elevated pressures (above 10 atmospheres). The use of such elevated temperatures and pressures is of course contemplated in con junction with the use of the aforementioned alcoholates. These methods of carrying out the reduction of the azoxy compounds to further states of reduction suffer the obvious disadvantages of expense attendant with the use of additional raw materials and expensive equipment such as pressure apparatus and the like. It has been proposed, in order to overcome such disadvantages, to employ certain promoters in the reaction mixture, together with an alkali alcoholate in order to effect reduction beyond the azoxy stage of compounds at a higher stage of oxidation than the hydrazo stage. US. Patents 2,684,358 and 2,684,359 illustrate disclosures of such promoters. The use of promoters in general, while permitting the reduction to proceed beyond the azoxy state at atmospheric pressure introduces, however, an undesirable additional alcoholates in the presence of a hydrogenation catalyst. The catalysts which are contemplated for use herein in conjunction with the alkali alcoholates may be nickel or any of the noble metal catalysts such as palladium, platinum, and the like in a supported or unsupported state. The platinum and palladium class of catalysts includes, in addition to these two elements, iridium, osmium, rhodium, and ruthenium. In addition to the free metals, one may employ the oxides and salts of these metals, and as indicated above, one may use these elements or compounds either unsupported as a finely divided powder, or supported on any suitable base well known in the catalyst art, such as charcoal, alumina, silica gel and the like. By employing such auxiliary catalytic agents with the metal alcoholates, it is possible to reduce aromatic nitro, nitroso, hydroxylamino and azoxy compounds to azo compounds and hydrazo compounds, and it further becomes possible to effect reduction of azo compounds to the hydrazo stage. The use of such catalytic agents not only permits a smooth and rapid reduction to the azo and hydrazo stages of reduction, but in addition, the processes are very economical due to the fact that the catalytic agents are separated in a simple manner from the reaction medium by filtration or centrifuging, and may be reused time and again until the activity thereof has diminished sufficiently to warrant either reactivation or regeneration by known means, or to discard them where economically feasible.

The amounts of the catalyst employed may be varied considerably. However, it is significant that extremely small amounts give effective conversion to stages of oxidation beyond the azoxy state. Such small amounts as from about mol per mol of reducible aromatic hydrogen compound are effective. Amounts up to about mol per mol of nitrogen compound may be used, the specific quantities to be employed being dependent upon the exact nitrogen compound being reduced and the specific conditions employed. Amounts larger than the aforementioned mol per mol of nitrogen compound are normally not necessaryor economically feasible.

The general process for carrying out the present invention involves the subjection of the reducible aromatic nitrogen compound to the reducing action of a metal a1- coholate in a reaction mixture which contains the aforementioned catalysts. The preferred form of the invention involves the addition of the reducible aromatic nitrogen compound to a suspension of the catalyst in the alcoholic medium while the latter is under reflux conditions, that is, at the boiling point of the resultant mixture. It is further preferred to add the reducible nitrogen compounds slowly to the alcoholate catalyst combination to obtain the highest yields. The isolation of the products resulting from the reduction of the starting reducible nitrogen compound may be done in any suitable manner well known in the art. Thus, the reduction product or products may be isolated by crystallization occurring upon cooling of the reaction mixture, and further purified by washing with water and alcohol to remove any other end products, and in particular, the sodium salts present. One may also distill the methanol, and then isolate the end products by crystallization. We have found it most expedient to carry out the isolation of the reaction products by forming two immiscible liquid phases, one an aqueous phase and the other an organophilic phase immiscible with the water phase, using a solvent such as toluene, benzene, or the like. By adding large quantities of water and one of these solvents to the reaction mixture and mixing thoroughly, the organophilic phase will extract the desired reduction product such as azobenzene, hydrazobenzene and the like, whereas the aqueous phase will contain the various Water soluble components and particularly the sodium salts present.

The amount of alkali and alcohol employed will vary depending upon the compound undergoing reduction, and also upon the desired end products. In all cases, an excess of these substances should be employed, that is, over and above the theoretical quantities necessary. Where it is desired to go to the hydrazo state of reduction, usually more alkali is necessary than Where the desired end product is the azo form. An excess of the alkali is also eflective in promoting an increase in the rate of the reaction.

The following examples will serve to illustrate the present invention without being deemed limitative thereof. In these examples, parts are by weight unless designated otherwise.

Example 1 89 parts of sodium hydroxide flakes are dissolved in 85 parts methanol. The solution is accompanied by the evo-. lution of heat, and after the solution has been cooled to room temperature, a paste of 2 parts of a 5% palladium on activated charcoal catalyst in 2 parts of water is added. The mixture is then brought to reflux and under this condition and with constant agitation, there is added over a period of 1 hour, 120 parts of azoxybenzene. After the addition, the resultant mixture is refluxed for 10 hrs. and then permitted to cool to room temperature. The catalyst is filtered oif, 200 parts of water and 70 parts of toluene are'then added, and after vigorous shaking, the layers are separated. The toluene layer contains the reduction product hydrazobenzene and a small amount of aniline, and also some 'unreacted azoxybenzene. This layer is washed several times with warm Water and then the hydrazobenzene content is rearranged to benzidine in the following manner. The toluene solution is added to 650 parts of water and the mixture cooled to to C. There is then added 175 parts of 20 B. hydrochloric acid and this mixture vigorously stirred and allowed to warm up to to C. The mass is maintained at the latter temperature for 4 hrs. and then after adding 840 parts of warm water, the mixture isheat-ed to 65 C. At this 7 period, 88 parts of 100% sulfuric acid. The mixture is then cooled to 50 C. and the resultant precipitate of benzidine sulfate is filtered off and washed acid free. The

roduct represents 63.5 parts of pure 'benzidine or 68.5% of the theoretical amount based on the azoxybenzene consumed, From the combined filtrate and washings of the benzidine sulfate cake there is obtained, after making the solution alkaline and steam distilling, a yield of 10.4 parts of aniline. This represents a yield of 11.2% based on the azoxybenzene consumed.

It will be observed that the total process for the con version to hydrazobenzene, that is, the reduction phase of the process, is carried out in 11 hrs. A similar process employing prior art promoters requires from three to four times this length of time to eifect a similar conversion.

Example 2 A solution of 275 parts of sodium hydroxide flakes in 1300 parts of methanol is prepared. After the solution has cooled down to room temperature, there is added 5 parts of a 5% palladium on activated charcoal catalyst similar to the one employed in Example '1. The mixture is then heated to reflux in a flask and then there is added over a period of 5 hrs., 184.5 parts of nitrobenzene. After the addition of the latter has been completed, the mixture is refluxed for an additional 2 hrs. Methanol is then distilled from the mixture until the flask temperature reaches C., at which time 400 parts of Water are added to the flask and then 300' parts of toluene. After vigorous stirring and filtration of the catalyst, the mixture is permitted to separate into two layers, the one aqueous and the other the toluene layer. The latter is then separated from the aqueous layer and contains the desired reduction products. This toluene solution is then treated in the manner described in Example 1, employing the same quantities of materials as was cited therein to convert any hydrazobenzene formed during the reduction to benzidine. There is finally isolated, again following the procedure of Example 1, 5.2 parts of benzidine which is equivalent to a yield of 3.7%. .From the filtrates and washes of the benzidine conversion there is obtained by making alkaline and steam distilling, a yield of 36.3 parts of aniline (26.1% of theory). The toluene layer present, after the benzidine conversion, is treated to recover any dissolved materials therein by distilling off the toluene. The residue consists of 70.4 parts of azobenzene having a freezing point of 65 C., indicating an extremely pure product. This yield represents a conversion of 51.7% of theory.

Example 3 A solution of 40 parts of sodium hydroxide flakes in parts of methanol is prepared. After the solution has cooled down to room temperature, there is added 1 pa t of 3% platinum catalyst supported on a silica base. This mixture is brought to reflux and over a period of 3 hrs. there is added 19 parts of nitrobenzene. After the addition of the nitrobenzene, the mixture is refluxed for an additional 4 hrs. 30 parts of water and 20 parts of toluene are then added to the mixture, and after vigorous stirring and removal of the catalyst by filtration, the toluene layer, after stratification, is separated from the aqueous layer. The toluene solution is then added to 100 parts of water and cooled to 0 C. 30 parts of 20 B. hydrochloric acid are added and the mixture, while under constant agitation, is permitted to warm up to 15 C. After 3 hrs. at this temperature, parts of water at 40 C. are added and the temperature further raised to 65 C. Upon standing, the mixture stratifies into an upper aoeaeso toluene layer and a lower aqueous layer. The latter is treated with 1 part of activated carbon and filtered while hot. To the filtrate, while at a temperature of 60 0, there is added slowly over a period of about 20 minutes, 15 parts of 100% sulfuric acid. The mixture is cooled to 50 C. and the resultant precipitate of benzidine sulfate is filtered oh" and Washed acid free. An exceptionally pure benzidine sulfate is obtained. 13 parts of benzidine are obtained which represents an overall theoretical yield of 87.5% of the nitrobenzene employed.

Example 4 The procedure of Example 1 is repeated except that the hydrogenation catalyst employed is a reduced and stabilized nickel on kieselguhr nickel). Similarly, as in this example, 2 parts of this catalyst are used. The yield of 'benzidine based on the amount of azoxybenzene consumed in the reaction is 65% of the theoretical yield. 10% aniline is also formed in this reaction.

The various catalysts contemplated in this invention are well known in the art and may be prepared in accordance with the following references:

Platinum black-Sabatier-Reid, Catalysts in Organic Chemistry. D. Van Nostrand Co., New York, 1922 Platinum oxide-Adams, Voorhees and Shriner, Organic Syntheses, Coll. vol. 1, p. 452. John Wiley & Sons, New York, 1932 Palladium on charcoalMannich and Thiele, Ber.

Deutches pharm. Ges. '26 36-48 (1916) Platinum on charcoal-Ellis, US. Patent 1,174,245

Nickel on kieselguhrCovert and Conner, J. Am. Chem.

Soc. 54 165 (1932) Platinum or palladium on Patent 1,111,502.

As pointed out above, the foregoing examples merely illustrate the present invention, and it is of course obvious that many changes and equivalent materials may be used in the process Without departing from the scope of the invention. Thus, the present invention may be applied to the reduction of other aromatic nitrogen compounds containing nitrogen in a reducible form as a nuclear substituent. Examples of such compounds include the following: the nitrotoluenes, 0-, m-, and p-nitrotoluene, 0-, m-, and p-nitrochlorobenzenes, nitrophenoli-c others such as o-nitrophenetole, 0-, m-, and p-nitroanisole, nitro aromatic carboxylic acids such as o-nitrobenzoic acid, pand m-nitrobenzoic acid, nitrobenzene sulfonic acids, 0-, m-, and p-nitroaniline and the like. As pointed out above, the end product or products obtained in carrying out the present invention depend on numerous factors such as starting material, that is, the state of oxidation of the nitrogen compound, and the amounts of alkali and alcohol employed. It is also possible to carry out the described processes at various temperatures and pressures. Thus, by employing an inert and miscible diluent in addition to the alcohol as the solvent, it is possible to exceed the temperatures normally obtained during reflux operations with metal alcoholates alone. pressures above atmospheric pressure, it is also possible to increase the reflux temperature. Usually, however, such elevated temperatures are neither necessary nor desirable because while the rate of reaction may be thereby increased, secondary and side reactions may reduce the yields of the desired reduction product. Further, while the examples illustrate the use of sodium hydroxide alumina-Schwarcman, U.S.

By the use of and methanol, these being preferred because of their low cost, other alkalis and alcohols may be employed, such use being well known in this art. Instead of the toluene employed in the examples to effect the isolation of the desired end products, any other equivalent solvent such as benzene, xylene and the like may be used.

Variations and modifications which will be obvious and apparent to those skilled in the art may be made in the procedure above described without departing from the scope and spirit of our invention.

We claim:

1. A method for reducing an aromatic nitrogen compound containing nitrogen in a reducible form as a nuclear substituent at a higher stage of oxidation than the hydrazo stage, said nuclear substituent selected from the group consisting of nitro, nitroso, hydroxylamino, azoxy and azo to a stage beyond the azoxy stage where the said nuclear substituent is a member of the group consisting of nitro, nitroso, hydroxylamino and azoxy, and to hydrazo where the said nuclear substituent is azo, which consists essentially in heating said aromatic nitrogen compound with a reducing mixture of an alkali metal hydroxide and a lower alcohol in the presence of at least about of a. mole of a hydrogenation catalyst selected from the group consisting of nickel and noble metal catalysts per mole of said aromatic nitrogen compound.

2. A method for reducing an aromatic nitrogen compound containing nitrogen in a reducible form as a nuclear substituent at a higher stage of oxidation than the hydrazo stage, said nuclear substituent selected from the group consisting of nitro, nitroso, hydroxylamino, azoxy and azo to a stage beyond the azoxy stage where the said nuclear substituent is a member of the group consisting of nitro, nitroso, hydroxylamino and azoxy, and to hydrazo where the nuclear substituent is azo, which consists essentially in heating said aromatic nitrogen compound with an alkali metal alcoholate in the presence of at least about of a mole of a hydrogenation catalyst selected from the group consisting of nickel and noble metal catalysts per mole of said aromatic nitrogen compound.

3. A method as defined in claim 1 wherein the hydrogenation catalyst is pr-esent in the amount of from about of a mole to about ,4 of a mole per mole of said aromatic nitrogen compound.

4. A method as definedin claim '1 wherein nitroben- Zene is reduced to azobenzene.

5. A method as defined in claim 1 wherein nitrobenzene is reduced to hydrazobenzene.

6. A method as defined in claim 1 wherein azoxybenzene is reduced to hydrazobenzene.

References Cited in the file of this patent UNITED STATES PATENTS 1,314,924 Andrews Sept. 2, 1919 2,012,234 Weiland Aug. 20, 1935 2,233,128 Henke et a1. Feb. 25, 1941 2,570,866 Sargent et al. Oct. 9, 1951 2,608,582 Huey et al. Aug. 26, 1952 2,688,040 Adams et a1 Aug. 31, 1954 FOREIGN PATENTS 17,914 France Oct. 13, 1913 68,490 Switzerland Mar. 16, 1915 552,147 Great Britain Mar. 24, 1943 881,-658 Germany July 2, 1953 

1. A METHOD FOR REDUCING AN AROMATIC NITROGEN COMPOUND CONTAINING NITROGEN IN A REDUCIBLE FORM AS A NUCLEAR SUBSTITUENT AT A HIGHER STAGE OF OXIDATION THAN THE HYDRAZO STAGE, SAID NUCLEAR SUBSTITUENT SELECTED FROM THE GROUP CONSISTING OF NITRO, NITROSO, HYDROXYLAMINO AZOXY AND AZO TO A STAGE BEYOND THE AZOXY STAGE WHERE THE SAID NUCLEAR SUBSTITUENT IS A MEMBER OF THE GROUP CONSISTING OF NITRO, NITROSO, HYDROXYLAMINO AND AZOXY, AND TO HYDRAZO WHERE THE SAID NUCLEAR SUBSTITUENT IS AZO WHICH CONSIST ESSNTIALLY IN HEATING SAID AROMATIC NITROGEN COMPOUND WITH A REDUCING MIXTURE OF AN ALKALI METAL HYDROXIDE AND A LOWER ALCOHOL IN THE PRESENCE OF AT LEAST ABOUT 1/1000 OF A MOLE OF A HYDROGENATION CATALYST SELECTED FROM THE GROUP CONSISTING OF NICKEL AND NOBLE METAL CATALYST PER MOLE OF SAID AROMATIC NITROGEN COMPOUND. 